this is a recent DSP receiver. i don't currently have a service manual for this, but most modern receivers like this behave similarly, so here's a general idea of what's happening. the receiver is going into protect mode. the microprocessor is detecting one of the following conditions in the receiver:
a) amp overload, one or more output devices shorted
b) DC offset in one of the amp outputs
c) power supply fault. one of the power supply voltages is too high, too low, or missing. on some makes of receivers (Yamaha for instance) this fault can be caused by loose screws on the chassis rear panel.
d) thermal fault, usually only happens to a cold amp if the temperature sensor is bad.
e) the microprocessor is "confused". can be cured by a reset to factory default settings

sometimes one of the above faults can be wrongly interpreted by the processor when an internal cable is unplugged.

any specific information on further troubleshooting such as special key codes to bypass the protection for further troubleshooting, or factory default reset key codes, will be in the service manual. some makes have a key code to bring up a protection history, so you can tell which of the above conditions caused the shutdown (Yamaha for instance) and some do not (Denon and Sony for instance).

G'Day,
I have developed the same problem on my tx-sv444. Just wondering if there was a cure. I have tried to reset the system and it returns the Clear command and then goes into Protect mode. I have also tried to initiate the test function and it begins then goes into protect mode. I do not want to spend a heap of money to get the unit fixed by a registered repairer if there is a quick fix out there.